Question: Which of these numbers is composite? ${21,\ 29,\ 67,\ 73,\ 89}$
Answer: A composite number is a number that has more than two factors (including 1 and itself). 29, 67, 73, and 89 each have only two factors. The factors of 21 are 1, 3, 7, and 21. Thus, 21 is the composite number.